Vibration when braking

[Tony]

A saturated topic in many clubs, but i wonder if wim can definitively explain this distressing phenomenon.

Question: Why do the brakes cause vibration.

[jammy]

A saturated topic in many clubs, but i wonder if wim can definitively explain this distressing phenomenon.

Question: Why do the brakes cause vibration.

 

 

warped disks/incorrectlly fitted pads.i;e retaner clips not put back properly.poss worn lower arms e.t.c???????

[scorps]

Drop links, badly weighted rims, incorrect geo etc etc etc!!

[Mark H.]

My brake disks have an outline of the pad, this seems to cause vibrations when braking from a faster speed. The car was used mostly as a weekend car so probably sat for a while inbetween use. They dont half give off brake dust either.

[Rich]

I've never had this problem but I always thought it was either warped discs or pads as said already...

[Tony]

A definitive reason.

 

THE NATURE OF BRAKING FRICTION

 

Friction is the mechanism that converts dynamic energy into heat. Just as there are two sorts of friction between the tire and the road surface (mechanical gripping of road surface irregularities by the elastic tire compound and transient molecular adhesion between the rubber and the road in which rubber is transferred to the road surface), so there are two very different sorts of braking friction - abrasive friction and adherent friction. Abrasive friction involves the breaking of the crystalline bonds of both the pad material and the cast iron of the disc. The breaking of these bonds generates the heat of friction. In abrasive friction, the bonds between crystals of the pad material (and, to a lesser extent, the disc material) are permanently broken. The harder material wears the softer away (hopefully the disc wears the pad). Pads that function primarily by abrasion have a high wear rate and tend to fade at high temperatures. When these pads reach their effective temperature limit, they will transfer pad material onto the disc face in a random and uneven pattern. It is this "pick up" on the disc face that both causes the thickness variation measured by the technicians and the roughness or vibration under the brakes reported by the drivers.

 

With adherent friction, some of the pad material diffuses across the interface between the pad and the disc and forms a very thin, uniform layer of pad material on the surface of the disc. As the friction surfaces of both disc and pad then comprise basically the same material, material can now cross the interface in both directions and the bonds break and reform. In fact, with adherent friction between pad and disc, the bonds between pad material and the deposits on the disc are transient in nature - they are continually being broken and some of them are continually reforming.

 

There is no such thing as pure abrasive or pure adherent friction in braking. With many contemporary pad formulas, the pad material must be abrasive enough to keep the disc surface smooth and clean. As the material can cross the interface, the layer on the disc is constantly renewed and kept uniform - again until the temperature limit of the pad has been exceeded or if the pad and the disc have not been bedded-in completely or properly. In the latter case, if a uniform layer of pad material transferred onto the disc face has not been established during bedding or break-in, spot or uncontrolled transfer of the material can occur when operating at high temperatures. The organic and semi-metallic pads of the past were more abrasive than adherent and were severely temperature limited. All of the current generation of "metallic carbon", racing pads utilize mainly adherent technology as do many of the high end street car pads and they are temperature stable over a much higher range. Unfortunately, there is no free lunch and the ultra high temperature racing pads are ineffective at the low temperatures typically experienced in street use.

 

Therefore - there is no such thing as an ideal "all around" brake pad. The friction material that is quiet and functions well at relatively low temperatures around town will not stop the car that is driven hard. If you attempt to drive many cars hard with the OEM pads, you will experience pad fade, friction material transfer and fluid boiling - end of discussion. The true racing pad, used under normal conditions will be noisy and will not work well at low temperatures around town.

 

Ideally, in order to avoid either putting up with squealing brakes that will not stop the car well around town or with pad fade on the track or coming down the mountain at speed, we should change pads before indulging in vigorous automotive exercise. No one does. The question remains, what pads should be used in high performance street cars - relatively low temperature street pads or high temperature race pads? Strangely enough, in my opinion, the answer is a high performance street pad with good low temperature characteristics. The reason is simple: If we are driving really hard and begin to run into trouble, either with pad fade or boiling fluid (or both), the condition(s) comes on gradually enough to allow us to simply modify our driving style to compensate.

[Tango]

Strangely enough, in my opinion, the answer is a high performance street pad with good low temperature characteristics. The reason is simple: If we are driving really hard and begin to run into trouble, either with pad fade or boiling fluid (or both), the condition(s) comes on gradually enough to allow us to simply modify our driving style to compensate.

...and just hope you are in a situation that enable you to modify your driving style to compensate when you encounter both fade and boiling fluid When I encountered both in my (original) 1275 Mini Cooper S it was after a 'spirited' drive and at a severe right hander at the bottom of a long and twisty steep hill. The overshoot of the bend dropped about 25 feet down an embankment and onto a railway line. I hit the brakes approaching the bend and foot went to the floor, no hydraulics at all Thank fcuk for Alec Issagonis and his design for that legendary go-kart as I managed to get round the bend when really I shouldn't have. After a brief stop the brakes were back to normal, but as my memory serves the adrenaline level and the heartbeat took a while longer.

 

Apologies for the war story...but regarding the relationship between pad and disk, the effect of drilled/dimpled/cross cut or grooved (or any combination of those) must therefore have a bearing on the braking efficiency. Presumably the selection of the right pad with these type of disc must be pretty critical to get the best out of the pairing?

[Tony]

Strangely enough, in my opinion, the answer is a high performance street pad with good low temperature characteristics. The reason is simple: If we are driving really hard and begin to run into trouble, either with pad fade or boiling fluid (or both), the condition(s) comes on gradually enough to allow us to simply modify our driving style to compensate.

...and just hope you are in a situation that enable you to modify your driving style to compensate when you encounter both fade and boiling fluid When I encountered both in my (original) 1275 Mini Cooper S it was after a 'spirited' drive and at a severe right hander at the bottom of a long and twisty steep hill. The overshoot of the bend dropped about 25 feet down an embankment and onto a railway line. I hit the brakes approaching the bend and foot went to the floor, no hydraulics at all Thank fcuk for Alec Issagonis and his design for that legendary go-kart as I managed to get round the bend when really I shouldn't have. After a brief stop the brakes were back to normal, but as my memory serves the adrenaline level and the heartbeat took a while longer.

 

Apologies for the war story...but regarding the relationship between pad and disk, the effect of drilled/dimpled/cross cut or grooved (or any combination of those) must therefore have a bearing on the braking efficiency. Presumably the selection of the right pad with these type of disc must be pretty critical to get the best out of the pairing?

How come?.... the 1275 was drums f/r if i recall correctly. This system is hardly susceptible to boiling the fluid.

[Tango]

How come?.... the 1275 was drums f/r if i recall correctly. This system is hardly susceptible to boiling the fluid.

 

It's supposed to be the short term memory that goes first The 850 cc standard Mini had drums all round, the 997 Cooper had discs on the front and drums on the rear but the front discs were smaller (they were really bad I understand) than the later 998 Cooper and the 1275 'S'.

My 'S' was very modified, lightweight bodyshell (not from rust), overbored stage two Janspeed motor, etc. but the only brake mods were harder front pads and rear shoes, supposedly anti-fade. It had wider Minilite alloys but they were still only 10" diameter so the discs were still tiny in size and bulk so no wonder they got hot enough to boil the brake fluid in the cylinders.

It drove exactly like a go-kart. The steering wheel connected you directly to the road with nothing in between so you felt every contour in the road. Great fun to drive, but looking back it really was a tin box with a wheel at each corner...hate to think of the safety aspect, just glad I survived driving it for four years

[Tony]

How come?.... the 1275 was drums f/r if i recall correctly. This system is hardly susceptible to boiling the fluid.

 

It's supposed to be the short term memory that goes first The 850 cc standard Mini had drums all round, the 997 Cooper had discs on the front and drums on the rear but the front discs were smaller (they were really bad I understand) than the later 998 Cooper and the 1275 'S'.

My 'S' was very modified, lightweight bodyshell (not from rust), overbored stage two Janspeed motor, etc. but the only brake mods were harder front pads and rear shoes, supposedly anti-fade. It had wider Minilite alloys but they were still only 10" diameter so the discs were still tiny in size and bulk so no wonder they got hot enough to boil the brake fluid in the cylinders.

It drove exactly like a go-kart. The steering wheel connected you directly to the road with nothing in between so you felt every contour in the road. Great fun to drive, but looking back it really was a tin box with a wheel at each corner...hate to think of the safety aspect, just glad I survived driving it for four years

 

A early 20th century Mini 1275 is long term memory.... Slightly faded i think! We can agree the machining of the brake configuration in those days was a bit.... Generous with tolerances, and not directly a good example of today's misgivings.

[jammy]

A definitive reason.

 

THE NATURE OF BRAKING FRICTION

 

Friction is the mechanism that converts dynamic energy into heat. Just as there are two sorts of friction between the tire and the road surface (mechanical gripping of road surface irregularities by the elastic tire compound and transient molecular adhesion between the rubber and the road in which rubber is transferred to the road surface), so there are two very different sorts of braking friction - abrasive friction and adherent friction. Abrasive friction involves the breaking of the crystalline bonds of both the pad material and the cast iron of the disc. The breaking of these bonds generates the heat of friction. In abrasive friction, the bonds between crystals of the pad material (and, to a lesser extent, the disc material) are permanently broken. The harder material wears the softer away (hopefully the disc wears the pad). Pads that function primarily by abrasion have a high wear rate and tend to fade at high temperatures. When these pads reach their effective temperature limit, they will transfer pad material onto the disc face in a random and uneven pattern. It is this "pick up" on the disc face that both causes the thickness variation measured by the technicians and the roughness or vibration under the brakes reported by the drivers.

 

With adherent friction, some of the pad material diffuses across the interface between the pad and the disc and forms a very thin, uniform layer of pad material on the surface of the disc. As the friction surfaces of both disc and pad then comprise basically the same material, material can now cross the interface in both directions and the bonds break and reform. In fact, with adherent friction between pad and disc, the bonds between pad material and the deposits on the disc are transient in nature - they are continually being broken and some of them are continually reforming.

 

There is no such thing as pure abrasive or pure adherent friction in braking. With many contemporary pad formulas, the pad material must be abrasive enough to keep the disc surface smooth and clean. As the material can cross the interface, the layer on the disc is constantly renewed and kept uniform - again until the temperature limit of the pad has been exceeded or if the pad and the disc have not been bedded-in completely or properly. In the latter case, if a uniform layer of pad material transferred onto the disc face has not been established during bedding or break-in, spot or uncontrolled transfer of the material can occur when operating at high temperatures. The organic and semi-metallic pads of the past were more abrasive than adherent and were severely temperature limited. All of the current generation of "metallic carbon", racing pads utilize mainly adherent technology as do many of the high end street car pads and they are temperature stable over a much higher range. Unfortunately, there is no free lunch and the ultra high temperature racing pads are ineffective at the low temperatures typically experienced in street use.

 

Therefore - there is no such thing as an ideal "all around" brake pad. The friction material that is quiet and functions well at relatively low temperatures around town will not stop the car that is driven hard. If you attempt to drive many cars hard with the OEM pads, you will experience pad fade, friction material transfer and fluid boiling - end of discussion. The true racing pad, used under normal conditions will be noisy and will not work well at low temperatures around town.

 

Ideally, in order to avoid either putting up with squealing brakes that will not stop the car well around town or with pad fade on the track or coming down the mountain at speed, we should change pads before indulging in vigorous automotive exercise. No one does. The question remains, what pads should be used in high performance street cars - relatively low temperature street pads or high temperature race pads? Strangely enough, in my opinion, the answer is a high performance street pad with good low temperature characteristics. The reason is simple: If we are driving really hard and begin to run into trouble, either with pad fade or boiling fluid (or both), the condition(s) comes on gradually enough to allow us to simply modify our driving style to compensate.

 

very interesting read that.slight diverse off topic here.{sorry}can warped disk also cause the car to shack/judder whilst drivin and under hard acceloration?????/

[Tony]

A definitive reason.

 

THE NATURE OF BRAKING FRICTION

 

 

very interesting read that.slight diverse off topic here.{sorry}can warped disk also cause the car to shack/judder whilst drivin and under hard acceloration?????/

There is no warped disc!... But a binding caliper acting on the pad matter during acceleration and generate vibration. even more so when the disc is hot.

[jammy]

A definitive reason.

 

THE NATURE OF BRAKING FRICTION

 

 

very interesting read that.slight diverse off topic here.{sorry}can warped disk also cause the car to shack/judder whilst drivin and under hard acceloration?????/

There is no warped disc!... But a binding caliper acting on the pad matter during acceleration and generate vibration. even more so when the disc is hot.

 

 

opps i missed that bit :rolleyes_anim: feal stupid now...ill get me coat.

 

 

so mite pay me to investigate the callipper's then.it doe's pull when brakin too which i beleave a binbing callipper will also cause this too

[Tony]

A definitive reason.

 

THE NATURE OF BRAKING FRICTION

 

 

very interesting read that.slight diverse off topic here.{sorry}can warped disk also cause the car to shack/judder whilst drivin and under hard acceloration?????/

There is no warped disc!... But a binding caliper acting on the pad matter during acceleration and generate vibration. even more so when the disc is hot.

 

 

opps i missed that bit :rolleyes_anim: feal stupid now...ill get me coat.

 

 

so mite pay me to investigate the callipper's then.it doe's pull when brakin too which i beleave a binbing callipper will also cause this too

 

Coat off!. Have a long drive, jack up the car release drive and handbrake then test for a bind by turning the wheels.